A conventional diaphragm is formed of, for example, metal material or an organic resin film. Examples of the metal material include aluminum. Examples of the organic resin film include a polyethylene naphthalate (PEN) film, a polyester (PET) film, and polypropylene foam including a foam layer. Furthermore, a diaphragm having an inorganic membrane on an organic resin film is known.
Note here that information on prior art literatures relating to the invention of the present application includes, for example, PTLs 1, 2, 3, and 4.